There is a strong interest in metal oxide semiconductor because of its high carrier mobility, light transparency and low deposition temperature. The high carrier mobility expands applications to higher performance domains that require higher frequency or higher current. The light transparency eliminates the need for a light shield in display and sensor active matrices. The low deposition temperature enables application to flexible electronics on plastic passivation layers.
The unique features of metal oxide semiconductors are: (1) carrier mobility is less dependent on grain size of films, that is, high mobility amorphous metal oxide is possible; (2) density of surface states is low and enables easy field effect for TFTs, this is contrary to covalent semiconductors (such as Si or a-Si) where surface states have to be passivated by hydrogen; and (3) mobility strongly depends on the volume carrier density. In order to achieve high mobility for high performance applications, the volume carrier density of the metal oxide channel should be high and thickness of the metal oxide film should be small (e.g. <100 nm and preferably <50 nm).
In thin film devices a gate dielectric is positioned over the portion of the metal oxide semiconductor layer that forms the channel for the device. The metal oxide semiconductor layer may include, for example, zinc oxide (ZnO), indium zinc oxide (InZnO), indium zinc gallium oxide (InZnGaO), etc. (see additional examples listed below). The gate dielectric is generally a material such as silicon oxide (SiO2), SiN, or the like. Generally, because of the deposition temperatures, etc. the metal oxide is amorphous and, preferably remains amorphous after processing. Because of the specific materials utilized, traps or trap states are formed at the interface between the metal oxide semiconductor layer and the gate dielectric layer. If interface trap states are deep in the bandgap, the trapping and de-trapping of carriers in the interface deep traps can manifest as a stability problem, i.e. a threshold voltage shift. In this instance “stability” is defined in terms of the threshold voltage of the TFT.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved metal oxide semiconductor device with improved stability.
It is another object of the present invention to provide a new and improved metal oxide semiconductor device with improved stability primarily due to reduction of interface traps.
It is another object of the present invention to provide a new and improved method of fabricating a metal oxide semiconductor device with a substantial reduction of interface traps.